This invention relates generally to thermal processing of semiconductor substrates including wafers, quartz reticles and LCD flat panel displays. More particularly, it relates to an integrated procedure of baking and subsequently chilling a substrate coated with photoresist.
The lithography processing sequence for producing integrated circuit lines on semiconductor substrates such as semiconductor wafers, quartz photomask blanks, and LCD flat panel displays, involves coating the substrate with a thin photoresist film. The film is subsequently exposed according to a predefined pattern by an electron beam or optical tool and then chemically developed to produce integrated circuit features. Several of the photoresist processing steps consist of baking the photoresist-coated substrate and then chilling it back to ambient conditions. The bake step is performed for several applications including evaporation of solvent, removal of standing wave effects, hardening of the photoresist, and accelerating chemical reactions for acid catalyzed photoresist. In order to achieve uniform processing, it is essential that spatial temperature variations are minimal throughout the entire trajectory.
The conventional manner of performing the baking and chilling of a substrate is shown in FIG. 1. An apparatus 20 includes a substrate 10 placed on a typically fixed-temperature hot plate 22, where it is heated up to a temperature typically between 70xc2x0 C. and 250xc2x0 C. for a period of time typically between 3 and 8 minutes for quartz photomask substrates and 30 and 120 seconds for a semiconductor wafer. The transient time, or time it takes the substrate to reach its steady-state processing temperature, is substantially longer for a quartz photomask blank than a semiconductor wafer because of its larger thermal mass (a typical quartz blank is 6 inches square by 0.25 inches thick, whereas a semiconductor wafer is 8 or 12 inches in diameter and less than 0.040 inches thick). After the substrate reaches its processing temperature and is held for a predetermined time, it is then mechanically moved to a fixed-temperature cold plate 24, where it is chilled to a temperature typically between 0 and 30xc2x0 C.
There are several disadvantages to this method of processing substrates. First, the movement of the substrate through the air from the hot plate 22 to the cold plate 24 causes the substrate to experience uncontrolled and nonuniform temperature fluctuations that persist during the entire cool down step. Second, uncontrolled temperature nonuniformities during the bake or chill steps may arise due to nonuniform convection currents over the substrate, especially with a square geometry of a quartz photomask substrate or a large 300 mm diameter of a silicon wafer. Third, the time required to move the substrate between the plates prevents the realization of very short thermal transition times between the bake and chill steps. Fourth, the procedure requires two distinct processing modules leading to an increased equipment footprint and the inability to integrate the module within other processing modules, such as the exposure tool. Fifth, the mechanical movement of the hot substrate between the plates may contaminate or otherwise damage the substrate. Sixth, the substrate is initially placed on a hot plate thereby making inaccuracies of the substrate""s temperature lowering mechanism inducing temperature nonuniformities that persist during the entire heating transient when one part of the substrate comes into proximity with the cold plate before another.
Referring to FIG. 2, a prior photoresist processing system 30 for silicon wafers, described in U.S. Pat. No. 5,431,700 by Sloan, discloses an apparatus where one of the plates, e.g., the hot plate 32 is placed upside down and directly above the other plate, e.g., cold plate 34. A lifting mechanism moves the substrate 10 only a short distance between the two plates. This approach reduces the nonuniform temperature induced by the wafer movement. However, the substrate still needs to be placed on constant temperature plates and the movement of the substrate is still required leading to nonuniform temperature deviations.
Referring to FIG. 3, another prior art photoresist processing system 40, described in U.S. Pat. No. 5,802,856 and incorporated herein by reference, includes a single integrated bake/chill plate 42. A passage is formed through the plate 42. To raise the temperature of the substrate 10, a hot fluid (e.g. between 70xc2x0 C. and 250xc2x0 C.) from a hot fluid supply 46 is introduced through passage 44 via a pipe 45. To lower the temperature of the substrate 10, cold fluid (e.g. between 0xc2x0 C. and 30xc2x0 C.) from a cold fluid supply 48 is introduced through passage 44. This system includes a resistive heat device 49 on the surface of the unit to improve temperature transients and steady-state holds. The resistive heater may be configured in a multizone arrangement to achieve improved temperature nonuniformity.
This system 40 eliminates the mechanical movement of the substrate during the temperature cycle and eliminates the placement of a cold substrate on a hot plate and vice versa. However, there are several disadvantages to the system. First, the entire apparatus is cycled in temperature. This procedure increases the energy requirements well beyond the theoretical minimum required to heat and cool the substrate. In addition, the hot fluid transportation through pipe 45 may pose a safety threat to personnel working close to the system in addition to nearby equipment that can become contaminated in case of a leak. Further, the fluid heat exchanger systems are bulky and expensive. In addition, the thin resistive heater placed on the surface of the exchanger, e.g. kapton, normally has operational limits of 200 to 250xc2x0 C. that prevent the realization of higher operating temperatures.
It is a primary object of the present invention to provide an improved method and apparatus for the spatial temperature control of material substrates. The method and apparatus may include, but is not limited to, thermal cycling. Such material substrates are quartz photomask blanks and silicon wafers. In particular, it is an object of the present invention to provide a method and apparatus that reduces the energy requirements, provides exceptional multizone temperature control of the substrate, improves reliability and increases the upper limit on temperature processing. An additional advantage of the present invention is the reduction of thermal deflection of the heating elements because of the length to thickness aspect ratio of the heating elements. Further advantages of the invention will be apparent from the following description and drawings.
This invention is concerned with a module for the temperature control of a material substrate. The module includes a plurality of independent thermally-conductive heating elements arranged in a planar fashion. An air gap or other suitable resistive material separates each of the thermally-conductive heating elements. The upper surface of the thermally-conductive heating elements is in thermal contact with the substrate. The temperature of the thermally-conductive heating elements is raised by a resistive heating element in thermal contact with it. A support structure holds the thermally-conductive heating elements in a fixed position. A cooling plate may be located in close proximity to and in the backside of the thermally-conductive heating elements. In a preferred embodiment, the cooling plate may rest on top of the support structure, thereby maintaining the support structure at a cold temperature. If maximum cooling is desired, the cooling plate is raised by a lift mechanism to contact the backside of the thermally-conductive heating elements to lower the substrate temperature. Alternatively, because the support structure is itself in thermal contact with the thermally-conductive heating elements, cooling at appreciable ramp-down rates can also be achieved without having to move the cooling plate at all.
An embodiment of the invention may include the following. The thermally-conductive heating element may consist of aluminum pieces with square-shaped, 1-inch by 1-inch, heads that are 60 mils thick. The head is thermally and mechanically coupled to a shaft. The shaft is hollowed with a cartridge resistive heater embedded within the opening. The resistive heater may be coupled to the shaft with a thermally conducting adhesive bond. The thermally-conductive heating elements are mounted on a support structure and placed in close proximity to each other (e.g. 20 mils apart). This proximity distance improves the thermal separation between the heating elements. In this fashion, the thermally-conductive heating elements form a square array of providing highly localized and spatially variable quantities of heat. It is noted that the discontinuity of the thermally-conductive heating elements is satisfactory to limit the cross-sectional heat conduction. Therefore, a thermally-resistive gap, which may involve the thermally-conductive heating elements in contact, is sufficient to achieve the thermal isolation. The heaters of the thermally-conductive heating elements are connected to a plurality of power supplies to achieve independent control over the spatial temperature distribution of the substrate. In a preferred embodiment, each heater is connected to its own independent power supply.
If thermal cycling is desired, a cooling plate may be positioned below the thermal-conductive heating elements and resting on the top of the support structure. The cooling plate has holes that can coincide with the location and dimension of the shaft of the heating elements. Further, the cooling plate has a passage through which cooling fluid passes. The cooling plate may be connected to a lifting mechanism that allows movement in the vertical direction. When cooling of the thermally-conductive heating elements is desired, the resistive heaters can be lowered in power or turned off. Alternatively, for faster cooling rates, the cooling plate can be positioned into thermal contact with the bottom surface of the thermally conductive heating elements. Depending upon the desired degree of uniformity and ramp-down rate, the square heating elements may or may not be energized during the ramp-down.
In a preferred embodiment of the invention, temperature is measured at each of the thermally-conductive heating elements. A temperature sensing device, such as an RTD or thermocouple, is embedded within the upper portion of the thermally-conductive heating elements. The temperature measurements are fed back to a controller that adjusts the power supplies that manipulate the resistive heating elements. The same controller may also be used if the cooling plate is present and ramp-down is desired.
In another embodiment of the invention for processing round substrates, such as semiconductor wafers, the head of the thermally-conductive heating elements can be shaped in such a way as to form an array of concentric elements that are also separated angularly, forming sectors of independent control. In another embodiment, the array of thermally-conductive heating elements may be machined from a single material piece, and grooves cut into the top and/or bottom surfaces of the single material piece to effectively achieve a plurality of thermally-conductive heating elements. Alternative resistive heating devices other than cartridge heaters may be utilized as well, for example, etched-foil heating elements, thermoelectrics or resistive heating elements embedded within or adhered to the square heating piece with leads extending through the shafts of the thermally conductive heating elements.